The proposed studies will evaluate the hypothesis that hyperprolactinemia inhibits cyclic LH release by acting through a hypothalamic neurotransmitter-LHRH mechanism. Experiments have been designed to answer three major questions. (1) Phasic LH release (preovulatory or steroid-induced) is accompanied, and presumably regulated by changes in norepinephrine (NE) an dopamine (DA) neuronal activity. Does high prolactin (PRL) affect these changes in neurotransmitter dynamics in such a way as to ultimately inhibit phasic LH release? 2) LH surges are regulated, in part, by the amount of LHRH released into portal blood and this release is accompanied by changes in tissue concentrations of LHRH in several hypothalamic structures known to contain cell bodies, axons and terminal boutons of these neurosecretory neurons. Does hyperprolactinemia affect LHRN concentrations in one or more discrete hypothalamic areas and/or the amount released into portal blood? (3) Serotonin (5HT) is known to increase PRL secretion and may be inhibitory to LH release. Does elevated PRL affect 5HT turnover rates in a way that could account for attenuated or absent LH su rges? Studies have been propsed using three experimental models: (1) the lactating rat, (2) the steroid-treated ovariectomized rat and (3) the cycling rat. Each of these models provides a controlled endocrine in which to examine the effect of endogenous PRL hypersecretion, or exogenously administered PRL, on the LHRH release mechanism. Sensitive neurochemical assays, in combination with microdissection methodology, allow for the measurement of changing neurotransmitter-LHRH dynamics in discrete nuclei, fiber pathways and terminal projection fields of the rat brain thought to be important in the neural control of PRL and LH release. Turnover rates of NE, DA and 5HT and LHRH concentrations will be measured in specific brain structures chosen because they are likely sites of synapse or neurohumoral contact between 5HT, DA and NE neurons and the cell bodies, axons, and/or terminals of LHRH-containing neurons. These brain parameters will be correlated with LHRH concentrations in portal blood and with the cyclic LH release in the presence and absence of elevated prolactin.